The present invention relates to a method of manufacturing a radial tire and an apparatus for practicing said method.
As is well known, a conventional method of manufacturing a radial tire is generally classified into a single stage type molding method and a double stage type molding method.
The single stage type molding method is practiced by way of the steps of first winding a material for a carcass layer on a drum device which is designed so as to expand and contract, placing bead cores at the predetermined position, allowing the drum device to expand so as to deform the cylindrical configuration into a trochoidal one, and then winding a breaker ply and a tread thereon or allowing the carcass layer to expand to the inner wall of a breaker-tread structure which is previously prepared.
On the other hand, the double stage type molding method is practiced by way of the steps of assembling a cylindrical carcass layer on a drum device in the first stage, said drum device being designed so as to expand and contract, removing the carcass ply from the drum device to transport the same, placing the carcass layer on another drum device in the second stage, said drum device being designed so as to expand and contract, deforming the cylindrical configuration of the carcass layer into a trochoidal one and then winding a breaker ply and a tread or allowing the carcass layer to expand to the inner wall of a breaker-tread structure which is previously prepared.
As is apparent from the above description, the single stage type molding method is free from the troublesome operation in the double stage molding method which comprises the step of displacing the cylindrical carcass layer from the drum device in the first stage to the drum device in the second stage, but it includes a number of problems because it is necessary to exactly locate a material to be assembled and effect firm assembling under pressure. Further, another disadvantageous feature of the single stage type molding method is that a distance between the adjacent cords is not distributed uniformly in the peripheral direction while the carcass ply is kept in an expanded state, because the carcass ply is expanded and comes in tight contact with the lower part of the bead cores, after the latter are placed on the cylindrical carcass ply.
Besides this a modified method is also proposed which includes the steps of previously assembling a carcass ply to a cylindrical configuration, transporting the assembled carcass ply to a drum device in the second stage with the aid of a transporting device, said drum device being designed so as to expand and contract, placing bead cores outward of the carcass ply, and then allowing the carcass ply to expand toward the lower portion of the bead cores so that they are brought in pressure contact with one another. However, it is found that the modified method has a drawback that during the pressure contact therebetween the cylindrical carcass is caused to expand and thereby fiber distribution density in the material loses its uniformity.
Further, another modified method is proposed which includes the steps of previously assembling a carcass ply to a cylindrical configuration, locating bead cores on the assembled carcass ply, allowing the latter to expand toward the lower area of the bead cores so that they are brought in pressure contact with one another while the bead cores are subjected to expansion, transporting the carcass ply-bead core assembled structure to a drum device in the second stage which is designed so as to expand and contract, and then effecting tire molding on said drum device. However, it is also found that the above second modified method has the same drawback as that of the preceding modified method, that is, fiber distribution density in the carcass ply loses its uniformity.
On the other hand, the double stage molding method is preferably practiced with the aid of a so-called high crown type drum which is designed so as to expand and contract for the purposes of ensuring that the material in the vicinity of the bead cores is hardly deformed or dislocated when a trochoidal configuration is achieved in the second stage as well as when final molding is effected in a vulcanizing die assembly.
However, it is found that a trouble or failure tends to occur with the high crown type drum during the step of winding a carcass ply, because it has an outer diameter which is substantially larger than the inner diameter of the bead cores.
Specifically, due to the arrangement that the bead cores are wrapped with the end portions of the carcass wound around an increased diameter a trouble or failure occurs that wrinkles appear in the vicinity of the bead cores when deforming the carcass inwardly in the radial direction (to achieve a reduced diameter). Another drawback is that incorrect location of the ply cords in the vicinity of the bead cores has an effect on incorrect location of the ply cords when deforming to a trochoidal configuration on the drum device in the second stage.
In order to obviate the drawbacks as described above to some extent a modified method is preferably employed which includes the steps of preparing a cylindrical carcass ply which has a diameter close to that of the bead cores, transporting it to a drum in the first stage, placing the bead cores on the carcass ply with the drum in the first stage expanded and thereby making a preparation prior to wrapping the bead cores.
It is true that the modified method has an advantage that no wrinkle appears. However, it is found that incorrect location of the ply cords occurs unavoidably at the shoulder parts of the drum, because sector pieces for constituting the drum surface are displaced as the drum is caused to expand.
The ply cords at the bead portions are held in a substantially correct location but they are not in a perfectly correct location. It should be noted that the incorrect location of the above-described type can not be remedied during the step of deforming to a trochoidal configuration.